1. Field of the Invention
The present invention relates to shape memory materials, and, more particularly, but not by way of limitation, to methods and an apparatus for utilizing a shape memory staple as an aid in operations including osteologic synthesis.
2. Description of the Related Art
Wire and staple fixation of bone have been used clinically for over 50 years. Cerclage techniques have commonly been used to encircle bone with wire to secure it to an adjacent bony segment. Variants of this technique initially involved taking a cerclage wire and bending it into a U-shape and inserting it into bone to provide intrabony fixation. As time progressed specialized staple with two of four legs were developed. These staple implants were fabricated from materials such as stainless steel and chromium-cobalt alloys and impacted into bone or placed so that the legs of the staple were inserted in drill holes thus minimizing surgery induced fracture at the implant site. As the use of staples expanded they were often contoured so as to match the anatomy of the site. Several examples of contoured staples exist such as the Richard's Krackow HTO, Krackow HTO Blage Staple, and Osteotomy Fixation Staple all from Smith & Nephew, Inc.; Semi-oblique and Offset Staple from Orthomed, S.A. and Tibial Osteotomy Staple, Memometal, Industries. These implants have been designed to contour to the bone surfaces where they are placed.
In the last 20 years, nickel-titanium staples have been used for their shape changing properties. These staples can be designed to retract their U-shaped legs so as to bring bone segments together. Like their predecessors they have also been contoured to match the anatomy of bone as illustrated by the Tibial Osteotomy Staple of Memometal Industries. This specific design uses a step in height to contour to the tapering portion of the tibia near the knee. This bony contour steps down from the width of the knee to the width of the shaft and can be well accommodated by the step staple. Though this staple has legs that deflect inwards during heating its stepped back does not change shape due to the undesired consequence of this shape change causing lifting of the back of the staple from the surface of the bone. This lifting would, in the application of the high tibial osteotomy, reduce the quality of fixation because the legs would partially pull out of bone and the back of the staple would be prominent and visible under the skin. These concerns have limited the applications of step staples to adapting to anatomical contours and taught against implants that change the shape of their back in all planes except the plane of the surface of the bone.
Accordingly the subject of this invention, a shape memory staple that provides restraint forces in multiple planes would be beneficial to surgeons, as well as persons requiring bone surgeries, because the shape changing back of the staple would not lift from bone such as in the high tibial osteotomy example but will compress the bone together while displacing them relative to one another.